The magnetoresistive speed measurement fluctuates upward to twice the normal speed

What does magnetoresistive mean? Can the speed not be measured accurately when using pulse counting? —— Stupid watch,,, I guess the crystal oscillator is broken, that’s why the timing is inaccurate. .

This issue may be related to the gear disc in some way. The American API has specific standard requirements, but many gear discs in China are not manufactured in accordance with these requirements; as a result, noise may occur during the measurement using a speed probe. By adding a 250-ohm resistor in the circuit and at the terminals of the speed probe, this noise can be eliminated.

1. Large compressors generally have 6–7 speed probes: 3 are used for speed control (the highest value from zones 2 and 3, plus 1 for individual control; the compressor stops operating in case of any fault). Mainly for high reliability; different strategies are adopted by various companies) ； 3-speed overspeed protection ； 1 for on-site speed display. Looking at the trend screenshots, the rotation speed at the other point seems normal; it is recommended to compare it with other rotation speed values. 2. If the issue lies solely in this speed, it could be one of the following: first, a fault with the MPU sensor itself (AI-TEK, DYNALCO, WOODWARD) ； II. The installation distance from the speed gear disc is inappropriate (refer to the instructions for the corresponding model of probe) ； III. Wiring issues: grounding, shielding, and wiring interference. It is recommended to exclude in segments. The probability of a fault in the probe itself is relatively high. 3. If there are problems with all the rotational speeds, it is more likely due to the reasons mentioned above. A mature crew should not be the first team; they generally don’t make such basic mistakes. Attachments P147~150 specify the requirements for the speed probe and gear disc.

This post was last edited by 6502855 on 2019-4-21 at 16:15. Thank you for your replies. This unit has a total of 7 probes: one is used as a spare, three are connected to the overspeed protector, and three are used for speed control. The rotation speed readings from the other 5 probes are normal. When the machine was shut down, fluctuations in rotation speed were observed, and it was suspected that the problem lay with the probes themselves. Therefore, the probes were replaced; they are of the American M.S.C brand. No corresponding instruction manual could be found. The installation clearance should be between 1 and 1.41 mm. Similar fluctuations persisted even after the machine was restarted, so it seems that the problems related to the probes themselves or their installation clearance can be ruled out. It is also suspected that it might be interference from the wiring and shielding, but strangely, the interference is regular, being twice the normal speed (as indicated by the other probes). I’m asking if anyone has encountered a similar problem.

The unit is an Elliott unit; the other 5 speed indicators are normal, so a problem with the gear disc can be ruled out.

1. If possible, replace the input wiring for the two different speed settings to determine more clearly whether it is an issue with the outer speed sensor and its wiring, or with the 3511 PI card channel on the inside； 2. Use an oscilloscope to check the waveform of the input signal, as well as its high and low voltage levels and frequency ； 3. I’ve never used M.S.C in the United States; are there any specific companies and models? In the past, AI-TEK, DYNALCO, and WOODWARD were used more often.

If this problem occurs only at low speeds, it is likely related to the assembly clearance or the probe itself. Try increasing the assembly clearance appropriately; this may reduce the sensitivity of the secondary instrument. If the problem cannot be resolved, the probe will need to be replaced. Regarding the cause of this phenomenon, I came across an explanation that seems reasonable; whether it’s correct or not is beyond me, but it’s offered here for reference only: A magnetoresistive sensor is essentially an inductor, and the change in magnetic flux as the gear passes by generates an induced electromotive force in the coil. When this back electromotive force interacts with a specific frequency passed by the gear, noise is generated. If this noise is large enough to be detected by the quadratic meter, the reading will double.

Was the probe that experiences fluctuations also replaced before? There is a magnetic shoe inside the probe, and the size of this shoe may differ between the new and old models, which in turn results in different voltage readings. Next time there’s a chance, the old probe that doesn’t experience fluctuations can be installed in that location where fluctuations occur, while the new probe can be installed elsewhere.